1. Field of the Invention
The present invention relates generally to a laser beam printer (LBP), and in particular, to a circuit and method for displaying a gray scale image in a black-and-white output device through multi-bit processing instead of conventional binary processing in a LBP in order to increase printing quality.
2. Description of the Related Art
In general, a multi-bit image, i.e., a gray scale image is converted to a binary bit map through binary image conversion prior to printing in an LBP.
FIG. 1 is a block diagram of a binary processing circuit in a conventional LBP. Referring to FIG. 1, the binary processing circuit is comprised of a half-tone table 102 for storing half tone values to be used for converting multi-bit gray scale image data to binary image data, a position controller 104 for controlling the half-tone table 102 to repeatedly read a half tone value corresponding to a threshold for a current pixel according to image size and the position of the pixel, and a comparator 100 for comparing the gray component of each pixel in an input image with the threshold for the pixel received from the half-tone table 102 and converting the gray component to binary data.
In operation, at the same time when image conversion starts, the position controller 104 generates a control signal including information about horizontal and vertical positions to allow a half tone value at a specific location to be read. Upon receipt of the control signal, the half-tone table 102 outputs a half tone value corresponding to a threshold for a pixel at the horizontal and vertical positions to the comparator 100. The comparator 100 compares the gray component value of the pixel in an input image with the half tone value received from the half-tone table 102. If the gray component value is greater than or equal to the half tone value, the comparator 100 outputs a logic high signal “1”, and if the gray component value is smaller than the half tone value, it outputs a logic low signal “0”. Then, the binary encoded output of the comparator 100 is printed on a sheet of paper by laser beams.
FIG. 2 (a) shows gray component values of the pixels in the input image, FIG. 2 (b) shows half tone values corresponding to thresholds for the pixels applied by the half-tone table 102, and FIG. 2 (c) shows binary encoded output of the comparator 100 after comparison between the pixels shown in FIG. 2 (a) and the thresholds shown in FIG. 2 (b).
As noted from FIG. 2 (c), the conventional binary processing method, which simply converts an input multi-bit image to a binary image, may decrease printing quality.
To solve this problem, a method has been explored in which the gray level of each image pixel is expressed by forming a half-tone cell with a plurality of dots in the form of a lattice and varying the number of dots to be on/off among the half-tone cell dots in order to convert a multi-bit image to a binary image while image quality is maintained at a desired level.
The above method, however, requires a large half-tone cell to express many gray levels and produces a very coarse printed image because dots within the half-tone cell are visible to the naked eye. Another problem is that a small half-tone cell also decreases the printing quality of an image because it increases the resolution of the image but allows a decreased number of gray levels to be expressed.